For more than 50 years, a method to replace malfunctioning or diseased tissue in the cardiovascular system has been an important area of development. Each year, over 275,000 replacement valves and 600,000 vascular grafts are implanted to correct damaged native structures. Complex pediatric and general cardiac surgeries also utilize tissue or synthetic patch material in reconstruction and repair. When considering materials for prosthetic applications, biological tissues offer some clear advantages over synthetic substitutes. Aside from the inherent biocompatibility, biological tissues possess “intelligent” elastic and mechanical properties that are unable to be mimicked by manmade material.
One biological tissue commonly used is pericardium. Pericardial tissue has historically been selected for cardiovascular devices due to its availability, inherent strength, and elastic properties. In addition to the desirable mechanical behavior, natural tissue often demonstrates superior fluid dynamic properties, and when compared to synthetic materials, requires less anticoagulation therapy.
In order to capitalize on and maintain the natural properties of pericardium, previous research has focused on improving the durability and biocompatibility of the material upon implantation. However, and despite significant advances, clinical experience continues to highlight the challenges of the prolonged use of pericardium implants in the cardiovascular system.
All natural tissue, including pericardium, can elicit an inflammatory and immune response from the host. To combat these events, pericardial tissue is commonly preserved in glutaraldehyde (GA), which chemically crosslinks the tissue's collagen molecules. This crosslinking process is effective at stabilizing the tissue against chemical and enzymatic degradation, as well as lessening the display of antigenic determinants. However, the crosslinked product has been associated with local cytotoxicity and, more importantly, severe calcification of the material that can over time lead to subsequent matrix deterioration and compromised mechanical properties.
The detailed pathways controlling calcification of cardiovascular tissue, both natural and prosthetic, are not explicitly understood, but as the most common pathology recorded in heart valve failures, it is certainly a process under high investigation. It is observed that chemically crosslinking pericardium damages and distorts the natural structure, destroys interstitial cells, and diminishes potential for viable cell inhabitation. The specialized matrix consisting of collagen, elastin and glycosaminoglycans that composes pericardium is responsible for allowing the tissue to accommodate the constant changes in shape and stress transfer, and is therefore essential to maintain. Damaging this natural structure and removing native cells results in a tissue that can no longer maintain or repair itself. It has been suggested in literature that calcification and eventual ossification is the result of insufficient or irregular repair of the tissue network. In culture, myofibroblasts have been shown to undergo phenotypic differentiation into the osteoblast like cells seen in calcified cardiac tissue, promoting calcification and bone type remodeling. Others suggest that the origin of bone cells in ossified valves in unknown, but their presence is confirmed in observation of excised heart valves and tissue.
Original damage leading to the irregular repair of the implanted tissue can be a result of mechanical stress, immune cell infiltration, or other pathologies, which complicates the investigation of the process. In one theory, responding immune cells are reported to secrete collagenase, among other proteolytic enzymes, which immediately begin to degrade the collagen network. It is further hypothesized by some that this initial proteolysis of crosslinked collagen debris creates foci for calcium deposition to initialize. Studies have shown a cooperative relationship between calcification of this type of tissue and the inflammatory response, enzymatic degradation, and microstructural failure (both independent failures and those associated with calcium deposits).
Regardless of the mechanism directly controlling the calcium deposits, GA treatment has been shown through both in vivo and in vitro accelerated testing to destroy the surface endothelium of the tissue, autolytically disrupt interstitial cells, alter natural collagen bundles, and destroy native GAGS. Despite the discrepancies in origin of the deterioration, a strong correlation between GA treated tissue and increased calcification suggests that the chemical fixation of the tissue further inhibits the appropriate remodeling.
Previous approaches to mitigate these limitations have focused on lessening the extent of crosslinking and resulting calcification by minimizing crosslinking time and by utilizing washing steps to removed excess GA. Although these successes reduce drawbacks of GA treated pericardium, any crosslinking process alters the natural tertiary structure of proteins that make up the tissue and threatens to negatively impact the ability for natural tissue remodeling and growth. Naturally derived materials, such as ECM structures including pericardium, have been shown to help define the microenvironment and signal the building of site appropriate functional tissue. ECM molecules represent a diverse set of structural and functional proteins and a variety of growth factors. Native binding sites from these molecules, as well as the formation of chemotactic cryptic peptides from parent molecules, can positively influence remodeling, recruit stem and progenitor cells, and modulate the immune response. These steps can ultimately determining the success of an implantable scaffold. If an ECM based hybrid biomaterial can be developed that promotes controlled regrowth of the injured site by avoiding crosslinking of the tissue, then the prosthetic can eventually be replaced with living tissue, and reduce subsequent surgical interventions.
The theory of calcification above suggests that if an alternative to GA can be developed that is effective at blocking enzymes and other immune activators, but avoids crosslinking of the tissue, then subsequent calcification of the material will be reduced. Thus, there is an ongoing and unmet need for improved compositions and methods for replacing and/or repairing malfunctioning or diseased cardiovascular and other tissues. The present disclosure meets this need.